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Since the earthquake in Japan the country has been having trouble with their nuclear energy plants and possible meltdown. The question is why aren’t they using lead to absorb the radiation?

Aerial view of the Fukushima I plant area in 1975

Schema of a BWR (boiling water reactor) Mark I, showing the layers or shields around the fuel rods

This is a complicated question, mainly because the issues with the Japanese nuclear reactors are also complicated. To make a long story short, there is probably already lead around and inside the reactors, as well as other neutron-absorbent materials like boron, concrete, and steel. But there are two reasons why lead isn't enough. First is that the lead might develop cracks that let radiation leak through. Second is that the lead isn't actually "removing" any radiation. Let me explain.

While lead, boron, some plastics, and a few other materials may do an excellent job of shielding radiation, they don't change the intrinsic characteristics of whatever is creating the radiation - in this case, the reactor fuel. The reactor fuel is releasing radiation in many forms: neutrons, alpha particles, gamma rays, etc. So long as the reactor fuel is present, radiation will be present. The materials inside the reactor which "absorb" neutrons specifically have done their job: the reactors are no longer "critical," that is, producing *more* radiation and energy. But there's leftover radiation (not just neutrons) that has to be stopped. So the question isn't one of "absorbing radiation" (the way one absorbs spilled milk with a sponge), but of shielding it.

In modern nuclear reactors, many safety features and interlocks are built-in, including shielding for the radiation that's produced. But when a catastrophe of the magnitude of the Japanese earthquake and tsunami hits a reactor, it's something so unbelievably bad that engineers don't take such things into account in their designs. A road, for example, is designed to withstand lots and lots of traffic, bad weather, and possibly other things like floods and fires, but it isn't designed to withstand the force of an airplane crashing into it - because such a thing is very unlikely to actually happen. The same is true of the Japanese reactors: they were designed to handle lots of things, but a 9.0 magnitude earthquake followed by a huge tsunami isn't one of those things, simply because it was very unlikely to ever happen. So even though the reactors were successfully shut down, stopping the continuous generation of new radiation (especially neutrons), the lead and other shielding around the reactors cracked a little bit, which is why small amounts of leftover radiation are leaking out. (All things considered, the result could have been a *lot* worse.)

You're right in thinking that more lead could be used to shield the broken reactors (after the Chernobyl accident, for example, the entire reactor building was encased in concrete to do just that), but at the moment the Japanese workers have a much bigger problem on their hands: hydrogen gas. Because the reactor cooling water systems failed (again, due to a catastrophic tsunami), the nuclear fuel got really hot, and it reacted *chemically* with other stuff inside the reactor, which created hydrogen gas. You probably already know that hydrogen gas is very dangerous, and very explosive. So the workers have been trying very hard to cool the reactor and get rid of the hydrogen, because that chemical poses a much larger threat than the radiation. Once the reactors are under control, in terms of temperature and chemical reactions, the workers can begin patching up the shielding to stop the radiation leak.